1. Field of Invention
This invention relates to sensing area coverage of a marking substrate.
2. Description of Related Art
Some currently available printing systems contain complex control and calibration loops, including internal process control and external color control loops. For example, printing systems disclosed in U.S. Pat. No. 5,471,313 and U.S. Pat. No. 6,236,474 contain levels 1, 2 and 3 control loops aimed at ultimately controlling the toner development process on the photoreceptor belt. These loops measure the state of the image on the photoreceptor by measuring the charge and developed mass per unit area for individual primaries, and actuate the charging and donor roll voltages to maintain the developability to some target set points. In addition, level 3 loops are designed to produce tone reproduction curves (TRCs) for each color separation (e.g., cyan, magenta, yellow and/or black) by measuring the developed toner mass on the photoreceptor belt.
A sensor such as an ETACS (Enhanced Toner Area Coverage Sensor) sensor system is used in level 2 and 3 loops to measure toner mass on the belt. The ETAC sensing head utilizes illumination by a single LED, which is tuned at a specific wavelength and uses a PIN diode as the detector.
Other printers use a similar multilevel control architecture with different kinds of sensor. The level 2 functionality is achieved using a single LED ADC (Automatic Density Sensor) sensor that measures the toner mass on the belt, and the level 3 functionality is achieved using a Color Patch-On-Paper (C-POP) sensor, which is a sensor that uses a single LED, that measures the toner density of paper-based individual primaries such as C, M, Y and K.